There is a wide variation in activity in the naturally occuring calcitonins with an approximate 40-fold range in biopotency. All calcitonins share common structural features. Each is 32 amino acids long with a C-terminal prolinamide and an N-terminal disulfide linked ring from position 1 through 7. Salmon 1 calcitonin, for example, has the following formula (Niall, H. D. (1969) Proc. Natl. Acad. Sci. USA 64, 771-778): ##STR1## Other calcitonins occuring in nature resemble salmon 1 calcitonin in varying degrees (Queener, S. F. and Bell, N. H. (1975) Metabolism 24, 555-567; Lasmoles, F. et al. (1985) FEBS Lett. 180, 113-116). In eel, ovine, bovine, porcine and chicken calcitonins the amino acid threonine occupies position 31. The same position 31 is occupied by the amino acid alanine in human and murine calcitonins. Salmon 2 calcitonin and salmon 3 calcitonin both have valine at position 31.
Eel calcitonin differs from salmon 1 calcitonin by having the amino acids Asp at position 26, Val at position 27 and Ala at position 29. Chicken calcitonin differs from salmon 1 calcitonin by having the amino acid Ala at position 2, Ser at position 3, Asp at position 26, Val at position 27 and Ala at position 29. Salmon 2 calcitonin differs from salmon 1 calcitonin by having Asp at position 15, Phe at position 22, Ala at position 29 and Val at position 31. Salmon 3 calcitonin differs from salmon 1 calcitonin by having Met at position 8, Asp at position 15, Phe at position 22, Ala at position 29 and Val at position 31.
The calcitonins of mammalian origin differ more markedly from salmon 1 calcitonin, as shown by the following comparison. The disulfide linkage between positions 1 and 7 is omitted from the following sequence for clarity.
______________________________________ Position: 1 2 3 4 5 6 7 ______________________________________ Salmon 1 Cys Ser Asn Leu Ser Thr Cys Human " Gly " " " " " Murine " " " " " " " Bovine " Ser " " " " " Porcine " Ser " " " " " Ovine " Ser " " " " " ______________________________________ Position: 8 9 10 11 12 13 14 ______________________________________ Salmon 1 Val Leu Gly Lys Leu Ser Gln Human Met " " Thr Tyr Thr " Murine " " " " " " " Bovine Val " Ser Ala " Trp Lys Porcine " " " " " " Arg Ovine " " " " " " Lys ______________________________________ Position: 15 16 17 18 19 20 21 ______________________________________ Salmon 1 Glu Leu His Lys Leu Gln Thr Human Asp Phe Asn " Phe His " Murine " Leu " " " " " Bovine " " " Asn Tyr " Arg Porcine Asn " " " Phe " " Ovine Asp " " " Tyr " " ______________________________________ Position: 22 23 24 25 26 27 28 ______________________________________ Salmon 1 Tyr Pro Arg Thr Asn Thr Gly Human Phe " Gln " Ala Ile " Murine " " " " Ser " " Bovine " Ser Gly Met Gly Phe " Porcine " " " " " " " Ovine Tyr " " " " " " ______________________________________ Position: 29 30 31 32 ______________________________________ Salmon 1 Ser Gly Thr Pro--NH.sub.2 Human Val " Ala " Murine " " " " Bovine Pro Glu Thr " Porcine " " " " Ovine " " " " ______________________________________
The structural features responsible for the increased potency of ultimobronchial calcitonins (e.g., salmon calcitonin) relative to calcitonins of mammalian origin have not yet been fully determined. However, a human calcitonin analog with Serine.sup.29 and Threonine.sup.31 substituents has been reported to be five times more active than the natural human calcitonin. (Maier, R. et al. (1974) FEBS Lett. 48, 68-71). In salmon 1 calcitonin serine appears at position 29 and threonine appears at position 31 and salmon 1 calcitonin is thirty to forty times more active than natural human calcitonin.
U.S. Pat. No. 3,849,388 mentions substitution analogs, including position 31 analogs in human calcitonin. The salmon 2 calcitonin analog with Val.sup.-, Ala.sup.29 and also Ala.sup.31 and Val.sup.29 have been claimed in U.S. Pat. No. 3,801,561. These analogs are of an equivalent potency as salmon 1 calcitonin. The introduction of Ser.sup.29 and Thr.sup.31 in salmon 2 calcitonin does not increase their activity relative to salmon 1 calcitonin (Maier, R. (1976) Calcif. Tiss. Res. (SUPPL) 317-320).